mwilliam , here's another idea , you can make the new project normally and just open the project folder
go to hooks file , then table name and put the following line in before inset and before update function , and it will sum them automatically

Hello Moh Youba,
i will explain the idea for you,
you will need to use after insert function in hook
after inserting in credit table ot after inserting in vestments table
you should first select sum of credits where customerid = logged customer and then use this value to update the credits field in cousomter table
and so on you can go through this logic for tota versement and for balance